Various synthetic vascular grafts have been proposed to replace, bypass or reinforce, diseased or damaged sections of a vein or artery. Commonly, tubular grafts have been formed from knitted or woven continuous filament polyester fiber (Dacron.RTM.) and from expanded polytetrafluoroethylene (PTFE).
The performance of a vascular graft is influenced by certain characteristics such as strength, permeability, tissue ingrowth and ease of handling. A graft should be sufficiently strong to prevent the sidewalls from bursting when blood is flowing through the device even at high blood pressures. A graft also requires the strength necessary to maintain the patency of the vessel lumen.
A graft sidewall must be sufficiently impervious to blood to prevent hemorrhaging as blood flows through the graft. Expanded grafts are inherently leak resistant. Woven and knitted grafts, on the other hand, may require sealing of the openings between adjacent interlacings to prevent blood leakage. One procedure, pre-clotting, a woven or knitted graft is immersed in the patient's blood and then allowed to dry until the interstices in the vascular fabric become filled with the clotted blood. Another common technique is to coat the graft with an impervious material such as albumin, collagen or gelatin.
Tissue ingrowth through the interstices of the graft is believed to nourish and organize a thin neointima lining on the inner surface of a graft, preventing clotting of blood within the lumen of the graft which could occlude the graft. A velour surface may be provided on the outer surface of a woven or a knitted graft to encourage tissue infiltration. The pore size of a graft also influences tissue ingrowth. Although larger openings facilitate tissue penetration, pre-clotting or coating of the graft may be adversely affected as pore size increases.
Ease of handling is another important feature of a vascular graft. A flexible and conformable graft facilitates placement of the prosthesis by the surgeon. Increased elasticity, particularly of woven grafts has been achieved by crimping the graft. Crimping also improves resistance to kinking when the graft is bent or twisted.
Woven and knitted grafts generally have been formed from continuous filament polyester yarns which typically are textured prior to fabrication to impart bulk and stretch to the vascular graft fabric. A technique known as false twist texturing has been employed which involves the steps of twisting, heat setting and then untwisting the continuous multifilament yarns, providing substantially parallel, wavy filaments. A false twist textured yarn 1 is illustrated in FIG. 1. Staple fiber polyester yarns have not been used because of the concern that individual staple fibers may detach from the fabric and be carried away by the blood flow, forming an emboli.
Graft selection for a particular application has therefore involved trade-offs and compromises between one or more of the above properties. Expanded grafts provide strong structures which are non-porous and impervious to blood leakage. The absence of pores, however, precludes tissue ingrowth. Expanded PTFE grafts also may be stiff and nonconforming which detrimentally affects handleability. Knitted grafts have attractive tissue ingrowth and handleability features. The porous structure of knitted grafts, however, requires that the graft be pre-clotted or coated to prevent hemorrhaging. Woven grafts are less porous than knitted grafts and may not require pre-clotting or coating. The tightly compacted weave structure, however, may provide a stiff prosthetic which is not as conformable or as easily handled as is a knitted graft.
Attempts have been made to enhance the strength, permeability, tissue ingrowth and handling characteristics of synthetic vascular grafts by forming the prosthesis from very thin fibers having less than one denier per filament ("micro-denier"). Representative are U.S. Pat. Nos. 4,695,280 and 4,743,250 which disclose artificial vascular grafts which have been formed from a combination of micro-denier filament yarns and larger yarns.